Brent Van Arsdell Limited Edition restoration

[Tom Booth]

I have not had any luck getting this engine to run by just applying heat and making timming adjustments, so it looks like it is going to require an overhaul.

Taking the engine apart, however, presents a challenge. I don't know if some parts are threaded, pressed on or glued, all I do know it it is not coming apart easily.

Is there anyone that knows how this engine is put together? I don't want to risk breaking it. (I've posted on the American Stirling company forum, but without any response so far).

To begin with, is this cap nut threaded or pressed on, or is it even a cap nut? It's hard to tell by just looking at it and it does not easily pull off or thread off by hand.

product.jpg (157.86 KiB) Viewed 16766 times

IMG_20220124_151900341.jpg (175.75 KiB) Viewed 16766 times

IMG_20220124_152000179.jpg (168.5 KiB) Viewed 16766 times

IMG_20220124_151951039.jpg (156.03 KiB) Viewed 16766 times

Maybe some WD-40 and heat it with a torch just a bit.

That's how I might normally loosen up a frozen nut, and wack it with a hammer a few times, but it would help to know in advance, what I'm doing. I don't want to break anything. It might be better to leave it alone.

[Tom Booth]

I thought finally, that heating up the nut, if that's what it was, with a torch, couldn't do any harm and should expanded the metal, losening it up. It had seemed impossible to remove without breaking something

With a little heat from the propane torch however, the brass retaining CAP, practically fell right off, with some goo bubbling out and smoking. It was actually rather loosely fitted to the post, but held in place with some kind of VERY strong glue or resin of some sort.

IMG_20220125_130139869.jpg (133.03 KiB) Viewed 16746 times

IMG_20220125_130202496.jpg (191.23 KiB) Viewed 16746 times

IMG_20220125_130059293.jpg (203.74 KiB) Viewed 16746 times

IMG_20220125_130222456.jpg (128.69 KiB) Viewed 16746 times

I assume other seemingly troublesome, impossible to remove parts can be freed up in the same way.

One warning however, whatever the stuff used to hold parts together on this engine, it gives off VERY unpleasant smelling, and probably toxic fumes when heated, so if anyone needs to restore one of these old engines, perform this operation in a well ventilated area.

There is also no guarantee that all the engines ever sold were assembled the same way.

[Tom Booth]

Having figured out how to get this thing disassembled, I have an idea for replacing the foam displacer, that frankly, IMO, seems a little cheap and tacky for such an apparently, quality work of kinetic art. At any rate, the existing displacer has seen it's day, is drooping and not functional. I want something that matches the rest of this engine in beauty, as well as functionality, so, what I have in mind is something like this:

If I can find or make some very thin hammered brass, or other textured brass. Very light and thin, something similar to one of these:

il_794xN.3099844073_gf20.jpg (210.21 KiB) Viewed 16741 times

And shape it into a regenerative displacer, something like this:

IMG_20220125_151533103.jpg (143.77 KiB) Viewed 16741 times

IMG_20220125_151553127.jpg (122.67 KiB) Viewed 16741 times

Air will have to mostly pass along and through the dimpled brass foils to get from top to bottom and back, the thin brass disk with the textured surfaces acting as a regenerative heat exchanger.

I don't know yet exactly what sort of magnetic center area the displacer has, or exactly how it functions, I know the engine has a central vertical pin to keep the displacer aligned and centered. This central mechanism, whatever it may be will have to be preserved and/or somehow incorporated and the new displacer will need to be approximately the same weight as the old foam one, so the magnetic lift system will still function as intended.

[Tom Booth]

The nuts and bolts holding the main body of the engine together, as it turns out, were very loose and came apart very easily by hand. The nuts were barely finger tight. Not even that really.

There also was no apparent seal. No glue or gasket or anything to make an air tight seal. I don't know why. Maybe the engine had been previously taken apart and whatever glue or gasket that might have been used removed. Maybe it never had any seal to begin with, maybe whatever small amount of glue there was dried out and came loose. I don't know, but obviously there was no chance of this engine running, given it's large size and no air tight seal at all. It will need to be completely taken apart and reassembled with some kind of actual gaskets to make it air tight

https://youtu.be/ySRFxaxVSpo

[Alphax]

Tom, it looks to me like an LTD (Low Temperature Differential) engine.

That being the case, they are extraordinarily sensitive to friction losses in the moving parts, as there is very little heat available to do work (the actual TD being only a few tens of degrees Kelvin).

So, keeping everything as free and frictionless as possible is critical to their success and could be a reason why some of the nuts were barely finger tight.

Don't be tempted to use oil.... it gradually thickens up and leaves tacky residues which slow things down. Graphite dust and loose fit are all you need.

More importantly, though, is the material of which the large, flat displacer is made of - it will not work if you use a metal because of the thermal conductivity being far too high given the small working temperature difference available to supply the heat. So what small amount of heat there is is lost in heat transfer through the metal displacer before it gets a chance to work. Equally important is that a thin metal displacer is far heavier than a cheap looking piece of foamy plastic and (because the engine has to lift it through each cycle before it drops down again) it will kill the engine instead.

Thats why cheap foam is used - it works (it doesn't conduct heat across the space between hot and cold plates and is light enough to be moved by the engine).

The very best material - by far - is EXTRUDED POLYSTYRENE SHEET (not expanded ploystyrene which is not the right material for this job, though it would actually work it would look rough). It comes in 3mm, 5mm and 6mm thick sheets which are very uniform and accurate with smooth, shiny outer surfaces - and it is extremely stiff (it will not sag when warm, but don't get it over 90 degrees C or it will gradually warp). It used to be sold as "Depron" but that seems to have gone now - it is still commonly used as underfloor insulation board (for laminate floor coverings) and is available at Lowes Home Improvements (at least it used to be).

Because it is extremely uniform in density with smooth top and bottom surfaces it actually looks stunning! Nothing cheap and nasty about it at all and you can get absolutely perfect machined edges by cutting it with a hot wire bow (that is a bit of a black art if you aren't familiar).

The best source of high quality EXTRUDED polystyrene sheet is any large supplier of RC Model Aircraft parts and HobbyKing in the USA (which is where I guess you are) stocks it under "modelling foam" (used as a balsa wood substitute for aircraft wings and fuselages).

Alternatively, Dollar Tree stores stock it covered in shiny paper (both sides) which you can remove by spraying it with Windex, soaking in hot water then painstakingly peeling off. One sheet of Dollar Tree Foamboard (which is its proper name - model builders even call it "DTF") is a lifetimes supply and will cost you just a dollar or two.

Trust me, it is the best material for the job BY FAR - I've been using it for years.

[Alphax]

Beautiful engine, by the way.............

[Alphax]

Tom,

If you were wanting to use your lovely engine with a heat source hotter than 90 degrees (it would make it run a lot faster), then there is a high temperature alternative to Depron. It is used by florists (to stick flower stems into for floral displays) and is called 'Oasis Block'. It is generally green (so you can't see it in the flower display) and is very easily cut and shaped - but don't breath the dust. It is good for much higher temperatures than depron.

[Tom Booth]

Tom, it looks to me like an LTD (Low Temperature Differential) engine.

(...)

More importantly, though, is the material of which the large, flat displacer is made of - it will not work if you use a metal because of the thermal conductivity being far too high given the small working temperature difference available to supply the heat. So what small amount of heat there is is lost in heat transfer through the metal displacer before it gets a chance to work. Equally important is that a thin metal displacer is far heavier than a cheap looking piece of foamy plastic and (because the engine has to lift it through each cycle before it drops down again) it will kill the engine instead.

It is a typical LTD "pancake" type design, though it is shown as operating on a lamp or alcohol burner in the original drawings, but then, it came with a foam displacer that obviously (to me anyway) could not survive the heat of a flame, even a small tea candle, so .

I don't think, for a $2,000 model engine, enough thought was put into these maters. It was apparently intended more as a beautiful work of art to sit on a shelf, not so much a practical working engine

I want it to be able to run - as a high temperature FLAME driven engine, as depicted in the original artistic rendering:

Resize_20220210_111244_4636.jpg (83.74 KiB) Viewed 16616 times

Your points regarding a brass or metal displacer are noted, but many high temperature engine work acceptably with metal displacers, and I think, in this case, it could be made to work.

The dimpled surface of the hammered metal foil could create a turbulent buffer layer of air, or "boundary layer" that will act as insulation. The air flow will be mostly horizontal, so there should be relatively little direct impact of air to metal.

The ability to increase heat input would likely compensate for any added weight, and I don't really think that a thin metal foil would necessarily be heavier than the original, much thicker foam rubber.

My reasoning could be wrong, but the appearance and quality matter, also the ability to withstand much higher heat than your typical "coffee cup" engine matters.

Robert Stirling's original model included long metal displacer. I think the long journey the air will have to make to get around and through such a large "pancake" displacer should function similarly.

Time will tell. Anyway, I think it is worth a try.

The absence of a good seal between the displacer chamber plates and wall, is a huge problem, and the loose nuts and bolts only add to the lack of a positive air tight seal. Tightening down the bolts, with a gasket, to get a better seal would not cause any friction whatsoever. Those are not moving parts of the engine. Or aren't supposed to be moving, though, as I received the engine, these parts were loose and moved easily. The engine could never operate without an air tight seal with the bolts securely tightened down.

[Alphax]

Hi Tom

Obviously fixing the seals is essential! I use a high temperature elastomer seal - I think we used to call it "gasket goo" decades ago (but it wasn't high tech stuff like it is today).

One thing I have done in the past that helps is to rub the cut ends of the glass cylinder on flat diamond "stones" (used for honing chisel and hand plane blades). You can get really nice flat and true square edges that take the minimum of sealer "goo" (just a thin smear that can't be seen on a lovely machine like yours).

Regarding displacer material, I realise you'd like it to look nice, but I think you'll struggle with metal. Why not use foam to get it to work in the first place, allowing you to fine tune the working engine, then see if it still runs as well if you swap to a metal displacer.

The displacer material in LTD Stirling engines like yours has been studied using amazingly well engineered, instrumented test engines and the published results show that metals (aluminium alloy) and non-metals (MDF/fibreboard) run very differently, with the metals doing significantly worse (less power, slower speed) than non-metal displacers. I think that tells you metals are more difficult to get to work, even though they may look nice!

The reference for that study is:-

https://www.researchgate.net/publicatio ... ing_engine

- it is a good read!

[Tom Booth]

BTW, I've recently noticed that there are double walled, vacuum insulated water bottles, not only in stainless steel, but also Titanium, which has some interesting properties as a potential metal for use in a Stirling engine.

Besides being very light weight, it also does not readily conduct heat, which as I understand it, is a problem when it comes to turning or machining the metal. The heat from friction of a cutting blade is held by the titanium, rather than dissipating, so more frequent cooling and a slower turning speed is required, If it gets extremely hot it could even ignite.

As a displacer at any normal flame temperature though? Being both lightweight, very rigid, and conducting heat very poorly, it might be a good option for this application.

The contrast of colors between brass and titanium might also look very nice.


https://youtu.be/JAyFLiG1hb0

Just beware, some water bottles advertised as "titanium" are titanium colored, not actual titanium.

Edit: cross posted, but a quick scan of that PDF does not turn up any mention of titanium.

[Alphax]

Tom, thin titanium sheet would be excellent as a displacer, if only you could work it! From what I understand, it is a difficult material to cut and shape, requiring specialist skills and specialist cutting tools (well, tools not available in my little workshop, any way!).

If you really have your heart set on a metal displacer (I see that you do!) then why not try a really thin stainless steel sheet? Grades like 314 have similar thermal conductivity to titanium (ie suitably low), though not quite as good - the real downside being stainless - even thin stainless - is heavier than titanium. And I wouldn't like to work with stainless much either as it is still tricky to cut and shape - certainly for me it would be!

I know copper alloys (brass etc) look attractive, are cheap and work easily but their thermal conductivities are marginal at best for LTD displacers - but definitely worth a try if your'e keen on the appearance thing! It would certainly be unique - I've never seen anyone succeed with a metal LTD displacer (foam is always the 'go to' material on the cheap little imported ones and Jim Larsen designs).

[Tom Booth]

Oh well, before returning to the forum, (I was out pouring concrete), I called a local machine shop to see if they could machine titanium.

Well, they could, theoretically, but don't often and are booked up for three months so couldn't do anything any time soon anyway, and...

They deal with a bulk supplier that has minimum requirements on supplies $500 minimum, yada yada ok whatever.

So I looked online and found this right off:

Resize_20220210_160313_3103.jpg (113.58 KiB) Viewed 16599 times

https://onlinemetalsupply.com/6al-4v-gr ... x-12-x-12/

$25. isn't much more than I paid for a sheet of thin silicone rubber, so I went ahead and ordered some, and with free shipping to my area, it might even be less.

.016 is mighty thin.

I've used very big titanium air nail guns that felt nearly weightless compared to any old steel air tool, so how heavy could a .016 disk of titanium be?

Turning the square sheet down to about a 5" disk on my mini lathe may, or may not present a problem, but I can't really imagine cutting through sixteen thousandths of an inch could be that difficult.

I guess I'll find out because I don't feel like waiting around for three or four months.

BTW, I'm not entirely fixated on using metal. Wood could look nice, probably. Some light weight heat proof ceramic could be an option. I still have several gallons of lightweight glass and ceramic microspheres I could add to clay and fire in my kiln, but that isn't set up yet.

I just don't want to go with something that is going to look cheap and "tacky", or won't hold up under actual use with high heat.

I've always wondered what kind of power output could be produced by a little LTD if it were made of material that could actually take some heat.

There really isn't much of the displacer to see, just the edge, mostly.

I don't yet even know if the clear part of this engine is glass, or just acrylic. If it is just plastic, that will have to be changed.

If need be I could cut a glass cylinder from a large glass bottle or jug, if I can find something the right diameter.

The way it looks, I suspect it is just acrylic but it is difficult to tell just by looking at it.

Resize_20220210_162540_0873.jpg (165.14 KiB) Viewed 16598 times

That rough edge could be poorly cut glass or the rough edge, or bubbles from cutting acrylic of cutting through it with heat, Still haven't gotten around to taking the thing apart any further as I'm still in the process of setting up a new workshop, but when I do, it should be obvious what it is. The edge does not look smooth. Another reason there isn't any good air tight seal. Not even close.

[Alphax]

Hi Tom

That is mighty thin! Well done - it will be interesting to see how that works out - if you have your heart set on a metal displacer, that sounds ideal.

[Tom Booth]

I made a mistake, the nail guns are made of magnesium which is much lighter than titanium.

Titanium is lighter than stainless steel, but heavier than aluminium.

Magnesium, though, is much more conductive of heat.

The heat conductivity of titanium is very very low, and titanium is stronger, so probably can be thinner.

I might just see if I can find some magnesium sheet metal, just for comparison.

Looking around, there are apparently many new very light weight, super strong alloys that have been discovered recently, "lighter than aluminium but stronger than steel".

I see very small thin magnesium sheets, about 100mm quite cheap, under $10, but finding something a little bigger, at least 150mm square for this engine seems elusive. Bigger, thicker and much more expensive, but just a small say 6" x 6" or 12" x 12" no.

The same company does not seem to deal in magnesium. Why I don't know. They have everything else.

[Alphax]

No worries!

Magnesium might be light, but is also a bit prone to catching fire, given half a chance!

I really hope your titanium displacer works - it is a great idea and well worth trying. Can you do a control experiment and compare it with a Depron displacer (it would be lighter and a lot lower thermal conductivity than even a 0.016" titanium one). Here are the numbers:-


For two 12" x 12" sheets, one in titanium (0.016" thick) and one in Depron (0.25" = 6mm thick:-

Volume of the Titanium sheet = 12 x 12 x 0.016 = 2.304 cu inch

Volume of Depron = 12 x 1 x 0.25 = 36.0 cu inch

Density of Titanium = 4480 kg per metre cubed

Density of Depron = 40 kg per metre cubed


Weight of Titanium sheet 12" x 12" x 0.016" = 6 ounces

Weight of Depron sheet 12" x 12" x 0.25" = 0.8 ounces


That is to say, whatever weight your titanium displacer turns out to be (perhaps about 1 ounce), the same diameter displacer in 6mm Depron would weigh less than one sixth as much (perhaps 0.15 ounce)


So - can you do both and compare them? It would be a brilliant experiment as no-one has ever compared super-thin titanium displacers to the usual foam ones! I know which one looks better though (hint: titanium all the way!).